Straight stitches of lock stitching may be divided into normal stitches as shown in FIG. 7, and hitch stitches as shown in FIG. 8. If those are mixed, the stitches are disordered as seen in FIG. 11. In general, it is desirable to form a series of normal stitches which will not cause untwisting of the upper thread.
The normal stitches and the hitch stitches are produced in dependence upon whether a needle drops on a left side or a right side of the lower thread drawn from a thread source in a loop taker, as illustrated in FIGS. 9 and 10.
When the needle drops on the right side of the lower thread, the normal stitches are formed as shown in FIGS. 9(a) to (g). When the needle drops on the left side of the lower thread, the hitch stitches are formed as shown in FIGS. 10(a) to (g).
Therefore, in order to drop the needle on the right side of the lower thread as shown in FIG. 9, the lower thread need be supplied at the left side of the loop taker leftward of a lateral elongate needle dropping hole 18a as seen in FIG. 12(a), because straight stitching is generally carried out with the needle dropped at the left side of the needle hole.
However, in this case if the zigzag stitching is carried out, the lower thread will pull out the upper thread too much when the lower thread is interlocked with the upper thread at the right side of the zigzag stitch in contrast to the normal zigzag stitch. This results from different stitching conditions at the right and left sides of the zigzag stitch as shown in FIG. 12(a). Namely, because of different distances from the lower thread supply point to the right and left sides.
The applicants' Japanese utility model Laid open application 56-23,074 (1981) discloses a mechanism for changing the position of the lower thread supply point when the zigzag stitching is carried out as shown in FIG. 12(b) so that the distances from the lower thread supply point to the left L and right R sides of the zigzag stitch are substantially the same. However, this mechanism is arranged in a narrow space between the needle plate and the loop taker, and its structure is rather complicated and manufacture is costly because of the need to meet space requirements.